Curable silicone compositions which cure in response to a hydrosilylation reaction are used for various applications since the compositions form cured products with excellent rubber-like properties such as hardness and elongation. Examples of known compositions include a curable silicone composition comprising: a straight-chain organopolysiloxane having at least two alkenyl groups in a molecule; an organopolysiloxane consisting of SiO4/2 units, ViR2SiO1/2 units, and R3SiO1/2 units (wherein, Vi is a vinyl group, and R are substituted or unsubstituted monovalent hydrocarbon groups which do not contain unsaturated aliphatic bonds); an organopolysiloxane having at least one silicon-bonded alkoxy group and at least two silicon-bonded hydrogen atoms in a molecule; and a platinum metal-type catalyst (see Patent Document 1); and a curable silicone composition comprising: a straight-chain organopolysiloxane having at least two alkenyl groups in a molecule; an organopolysiloxane consisting of SiO4/2 units and R′(CH3)2SiO1/2 units (wherein, R′ is an alkenyl group or a methyl group) and having at least three alkenyl groups in a molecule; an organopolysiloxane consisting of SiO4/2 units and R″(CH3)2SiO1/2 units (wherein, R″ is a hydrogen atom or a methyl group) and having at least three silicon-bonded hydrogen atoms in a molecule; and a platinum metal compound (see Patent Documents 2 and 3).
Such a curable silicone composition forms a transparent cured product without generating byproducts and is therefore suitable as a sealing agent for an optical semiconductor device. However, when used for a long period of time in an environment at 200° C. or higher, cracks form in the cured product, which leads to the problem of diminished adhesion with the substrate, diminished sealing performance, and diminished transparency.
On the other hand, adding a cerium-containing organopolysiloxane is a well-known method of improving the heat resistance of a curable silicone composition (for example, see Patent Documents 4 and 5). However, since cerium-containing organopolysiloxanes give off a yellow color, they have not been used in curable silicone compositions for optical semiconductor devices requiring transparency.